Differential changes in perceived contrast following contrast adaptation in humans

Perceived contrast is reduced after prolonged exposure to a textured pattern (contrast adaptation). The size of this effect is dependent on the relationship between the adapting contrast and the test contrast. It is generally accepted that the greatest reductions occur when the adapting contrast is much higher than the test contrast. Here this relationship was examined for a wide range of spatial frequencies. The results show that the effect of the adapt/test ratio on perceived contrast following contrast adaptation is highly spatial frequency dependent. At high spatial frequencies >1 cpd perceived contrast was reduced for all adapting contrasts, which is consistent with other studies. However, at low spatial frequencies (<1 cpd) the perceived contrast was actually above veridical perception when the adapting contrast was lower than the test contrast. This finding has not been previously reported and has important implications for models of contrast perception.